In a communications network, it is often desirable to identify and distinguish one transmitter from other transmitters operating within the network. For example, in the radio telephone industry, a cellular telephone system utilizes an electronic serial number (ESN) and a mobile telephone identification number (MIN) to provide a unique identification for each cellular telephone. To place a phone call, the individual subscriber or other authorized user of a cellular telephone dials a telephone number and presses the "Send" button. In response, the cellular telephone transmits its ESN and MIN to the cellular system so the individual subscriber can be charged for the telephone call.
Unfortunately, unscrupulous individuals illegally operate cellular telephones by "cloning" the ESN and MIN of a valid subscriber's telephone in order to obtain illegal access to the cellular system without paying for the service. A pirate can obtain the ESN and MIN of a cellular telephone by monitoring a transmission of the cellular telephone, and then programming them into another telephone for illegal use. Thus, the mere transmission of the authentic ESN and MIN are inadequate to protect a cellular telephone system from illegal use by the pirates.
In an effort to provide additional security, some cellular systems and other wireless services, authenticate cellular telephones based on the transmission of data by the cellular telephone during a call set-up process. Rather than identify the cellular telephone by its ESN and MIN alone, the system identifies a cellular telephone by its transmission characteristics. In this manner, the cellular system can reject calls from cloned cellular telephones even when those cellular telephones transmit valid ESN and MIN numbers. For example, in U.S. Pat. No. 5,005,210 issued to Ferrell on Apr. 2, 1991 ("the Ferrell patent"), incorporated herein by reference, a system is described that analyzes certain transmitter characteristics in an effort to identify the transmitter type. The system in the Ferrell patent analyzes the manner in which the modulator makes a transition to the designated carrier frequency. This transient reference waveform is used to identify the type of transmitter. The transmission characteristics of the reference waveform can be processed in different manners to create a "fingerprint" of the individual transmitter.
While the Ferrell patent describes one class of transmission characteristics that can be used as a fingerprint, other fingerprint characteristics are also known in the art. For example, U.S. Pat. No. 5,420,910 to Rudokas on May 30, 1995 ("the Rudokas patent"), incorporated herein by reference, describes an identifier, such as a radio frequency signature, that can be used to positively identify a valid cellular telephone or a known fraudulent telephone. Other types of signature or fingerprint authentication systems are also known in the art. However, all fingerprint authentication systems require at least one reference waveform, known to be generated by the authentic cellular telephone, which is used as the fingerprint itself or is used to derive a fingerprint. A fingerprint as used herein includes reference waveforms transmitted by a transmitter as well as the various transmission characteristics derived from the waveforms.
An effective fingerprint identification technique depends largely on the existence and quality of a database of stored fingerprints. In the Ferrell patent, a database of valid reference waveforms are collected by waiting for the transmitters to turn on. Not surprisingly, this type of a passive collection process may take a very long time to develop due to the unpredictability of the turn-on behavior of the transmitters. Moreover, such a passive collection process is impractical in a communications network such as a cellular telephone system in which there are millions of cellular telephones whose reference waveforms need to be collected.
It is therefore desirable to provide a system and method for rapidly collecting reference waveforms from a large number of transmitters within a relatively short period of time. The present invention provides this and other advantages, as will be illustrated by the following description and accompanying figures. In the figures, like reference numerals indicate similar components.